(1) Field of the Invention
This invention relates to a method and apparatus for determining an endpoint in a chemical-mechanical process for polishing the surface of a semiconductor wafer. More specifically, the invention is directed to the method of endpoint detection when polishing to remove a first layer of material and to stop the removal process when a second layer of material is exposed.
(2) Description of Related Art
In the fabrication of semiconductor components, metal conductor lines are formed over a substrate containing device circuitry. The metal conductor lines serve to interconnect discrete devices, and thus form integrated circuits. The metal conductor lines are further insulated from the next interconnection level by thin films of insulating material deposited by, for example, CVD (Chemical Vapor Deposition) of oxide or application of SOG (Spin On Glass) layers followed by fellow processes. Holes formed through the insulating layers provide electrical access between successive conductive interconnection layers. In such wiring processes, it is desirable that the insulating layers have a smooth surface topography, since it is difficult to lithographically image and pattern layers applied to rough surfaces.
Recently chemical-mechanical polishing (CMP) has been developed for providing smooth insulator topographies. CMP can also be used to remove different layers of material from the surface of a semiconductor wafer. For example, following via hole formation in a dielectric material layer, a metallization layer is blanket deposited and then CMP is used to produce planar metal studs. Briefly, the CMP processes involve holding and rotating a thin, flat wafer of the semiconductor material against a wetted polishing surface under controlled chemical, pressure, and temperature conditions. A chemical slurry containing a polishing agent, such as alumina or silica, is used as the abrasive material. Additionally, the chemical slurry contains selected chemicals which etch various surfaces of the wafer during processing. The combination of mechanical and chemical removal of material during polishing results in superior planarization of the polished surface. In this process it is important to remove a sufficient amount of material to provide a smooth surface, without removing an excessive amount of underlying materials. Therefore, a precise polishing endpoint detection technique is needed.
In the past, endpoint has been detected by interrupting the CMP process, removing the wafer from the polishing apparatus, and physically examining the wafer surface by techniques which ascertain film thickness and/or surface topography. If the wafer does not meet specifications, it must be loaded back into the polishing apparatus for further planarization. If excess material has been removed, the wafer may not meet specifications and will be substandard. This endpoint detection method is time consuming, unreliable, and costly. Therefore, numerous improvements to endpoint detection during CMP have been invented, as shown in the following patents.
U.S. Pat. No. 5,234,868 entitled "Method For Determining Planarization Endpoint During Chemical-Mechanical Polishing" granted Aug. 10, 1993 to William J. Cote describes a monitor structure surrounded by a moat. The moat causes polish removal to proceed faster at the monitor structure than at regions not surrounded by a moat. Polishing proceeds until the top of the monitor structure is exposed and results in a layer of planarized insulation above the metal pattern not surrounded by a moat. Visual inspection is employed to determine exposure of the top of the monitor structure. Alternately, monitoring is done electrically by detecting an electrical connection between the top of the metal monitor structure and the polishing pad.
U.S. Pat. No. 5,240,552 entitled "Chemical Mechanical Planarization (CMP) Of A Semiconductor Wafer Using Acoustical Waves For In-situ End Point Detection" granted Aug. 31, 1993 to Chris C. Yu et al directs acoustical waves at the wafer during CMP and through analysis of the reflected waveform controls the planarization process.
U.S. Pat. No. 5,308,438 entitled "Endpoint Detection Apparatus and Method For Chemical/Mechanical Polishing" granted May 3, 1994 to William J. Cote et al describes an endpoint detection method in which the power required to maintain a set rotational speed in a motor rotating the substrate is monitored. Endpoint is detectable because the power required to maintain a set rotational speed in a motor rotating the substrate significantly drops when the difficult to polish layer is removed.
U.S. Pat. No. 5,337,015 entitled "In-situ Endpoint Detection Method and Apparatus for Chemical-Mechanical Polishing Using Low Amplitude Input Voltage" granted Aug. 9, 1994 to Naftali E. Lustig et al utilizes electrodes built into the polishing pad, and a high frequency, low voltage signal, and detection means as a method for measuring the thickness of a dielectric layer being polished.
U.S. Pat. No. 5,413,941 entitled "Optical End Point Detection Methods in Semiconductor Planarizing Polishing Processes" granted May 9, 1995 to Daniel A. Koos et al describes a method for endpoint detection for polishing by impinging laser light onto the substrate being polished and measuring the reflected light. The intensity of the reflected light is a measure of the planarity of the polished surface.
U.S. Pat. No. 5,196,353 entitled "Method For Controlling a Semiconductor (CMP) Process By Measuring a Surface Temperature and Developing a Thermal Image of the Wafer" granted Mar. 23, 1993 to Gurtej S. Sandhu et al describes the use of infrared radiation detection to measure the surface temperature of a semiconductor wafer during a polishing process. Sudden changes of temperature at the wafer surface during the polishing process can be used to detect an endpoint.
The present invention is directed to a novel method for in-situ endpoint detection during a chemical/mechanical planarization (CMP) process using infrared monitoring of the temperature of the polishing pad. An abrupt change in temperature of the polishing pad is a result of a change in friction between the pad and the polished surface when, for example, one layer is removed and another layer is contacted by the polishing pad. The method provides a novel, inexpensive means to detect endpoint during CMP and is easily implemented on state-of-the-art polishing apparatus.